Cut the fish fillet into 1-½ inch pieces. Rinse once under cold water. Pat dry and place the fish pieces in a bowl. Marinate the fish with turmeric and salt and set it aside for at least fifteen minutes.
Meanwhile, blend onions, garlic, coriander seeds, chili powder, and cilantro to a fine paste with a little water (about 1 tbsp).
In a heavy bottom pan, heat oil and add the sliced garlic pieces to the hot oil. Once the garlic turns golden brown, add the sliced onions and green chilies. Sauté until the onions are soft. Then add the blended spices and fry it for a few seconds.
Add ¾ cup of coconut milk and 1 cup of water. Mix it well. Adjust the water if you think it is too thick. Bring it to a boil, and gently add the fish pieces along with the kokum or lemon juice. Let it simmer for 3-4 minutes on medium heat or until the fish is just turning opaque. Garnish it with the remaining coconut milk and serve hot with steamed rice.